This application claims the benefit of Korean Patent Application No. 2001-25388, filed on May 10, 2001, under 35 U.S.C. § 119, the entirety of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a method of fabricating a liquid crystal cell.
2. Description of Related Art
Recently, liquid crystal display (LCD) devices which are light and thin, and posses low power consumption characteristics are used in office automation equipment, video units and the like. Such LCDs typically use a liquid crystal (LC) interposed between upper and lower substrates with an optical anisotropy. The upper substrate, referred to as a color filter substrate, usually includes common electrodes and color filters. The lower substrate, referred to as an array substrate, includes switching elements, such as thin film transistors (TFTs) and pixel electrodes. Since the LC has thin and long LC molecules causing an alignment direction of the LC molecules, the alignment direction of the LC molecules can be controlled by applying an electric field to the LC molecules.
A brief explanation of a conventional liquid crystal cell manufacturing process and its operation will be discussed for better understanding of the present invention.
Common electrodes and pixel electrodes are formed on upper and lower substrates, respectively. A seal is then formed on the lower substrate. The upper and lower substrates are then bonded together using a seal such that the common electrodes of the upper substrate and the pixel electrodes of the lower substrate face each other, forming liquid crystal cells. Liquid crystal material is then injected into those cells through injection holes. The injection holes are then sealed. Finally, polarizing films are attached to the outer surfaces of the upper and lower substrates. The pixel and common electrodes generate electric fields that control the light passing through the liquid crystal cells. By controlling the electric fields, desired characters or images are displayed.
The liquid crystal cell process has few repeated steps compared with the TFT process or the color filter process. The whole process can be divided into the steps of forming the orientation film, forming the cell gap, injecting the liquid crystal and cutting the liquid crystal cell.
FIG. 1 is a flow chart illustrating a fabrication process of a conventional liquid crystal cell.
At step ST1, an initial cleaning is performed after an upper substrate and a lower substrate are prepared. This step is for eliminating the impurities on the substrate before forming an orientation film.
At step ST2, the orientation film is formed on the upper and lower substrates. This step includes deposition and rubbing processes of the orientation film or polymer thin film. The formation of the orientation film enables the liquid crystal to operate normally by the uniform orientation of the liquid crystal molecules and is needed for the uniform display property. The most important part of this step is to deposit the orientation film uniformly over the wide area. A polymer compound of the polyimide family is widely used for the typical orientation film, and the deposited polyimide thin film polymer compound becomes the orientation film through the preliminary drying and the hardening process. The rubbing process scours the orientation film along one direction with the rubbing cloth and the liquid crystal molecules align along the rubbing direction.
At step ST3, a seal printing and a spacer deposition on the substrates are performed. The seal patterns form cell spaces that will receive the liquid crystal material and prevent the interposed liquid crystal material from leaking out of the completed liquid crystal cell. The seal patterning is the process of patterning a thermosetting plastic mixed with glass fiber.
A screen-print method is widely used for this process. The next process is spraying the spacers. The spacers have a definite size and act to maintain a precise and uniform space between the upper and the lower substrates. Accordingly, the spacers are placed with a uniform density on the substrate using either a wet spray method, in which the spacers are mixed in an alcohol and then sprayed; or a dry spray method, in which only the spacers are sprayed.
At step ST4, the upper and lower substrates are aligned and attached. The alignment margin, which is less than a few micrometers, is determined by the substrate design. If the upper and lower substrates are aligned and attached beyond the alignment margin, light will leak out such that the liquid crystal cell cannot adequately perform its function.
At step ST5, the liquid crystal cell fabricated through the previous four steps is cut into unit liquid crystal cells. Generally, after a plurality of unit liquid crystal cells are formed on a wide glass substrate, the liquid crystal cell is divided into the plurality of unit liquid crystal cells. The cutting process typically includes a scribing process using a diamond pen to form cutting lines on the substrate, and a breaking process for separating by force the substrate along the scribed lines.
At step ST6, the liquid crystal material is injected into the unit liquid crystal cells. Since each unit liquid crystal cell is several hundred square centimeters in area but has only a few micrometers gap between the substrates, a vacuum injection method using a pressure difference is effectively and widely used. Generally, since injecting the liquid crystal material into the unit liquid crystal cells takes the longest manufacturing time, for manufacturing efficiency, it is important to have optimum conditions for the vacuum injection process.
A sealing process, which prevents the liquid crystal material from leaking through the injection hole, is performed after the injection.
After the injection and sealing processes, an inspection process and a grinding process are performed. Then polarization plates are attached on the outer surfaces of the upper and lower substrates so that the liquid crystal cell can be completed.
However, in this process of manufacturing a liquid crystal cell, the total turnaround time (TAT) becomes extended due to the time required for the liquid crystal injection, and thus the fabrication efficiency and production yields are decreased.
To solve these problems, a dispenser method, in which the liquid crystal materials are distributed to the substrate by a syringe, is suggested. The deposition process of the liquid crystal materials by the dispenser method is performed between steps ST3 and ST4 of FIG. 1. In the dispenser method, since the liquid crystal layer is directly formed onto the substrate, the total TAT can be decreased due to the reduction of time for the deposition process of the liquid crystal material.
FIG. 2 is a schematic view showing a deposition process of liquid crystal materials by a conventional dispenser method.
In FIG. 2, liquid crystal materials 14 are dispensed from a syringe 16 onto an interior of a seal pattern 10 of a lower substrate 12 by a point dotting method. The syringe 16 is filled with liquid crystal materials 14 and spaced apart from the lower substrate 12.
After this deposition process, an upper substrate is temporarily fixed to the lower substrate with an assembly and the upper and lower substrates are pressed so that the dispensed liquid crystal materials can be spread to the entire substrates. Then the upper and lower substrates are attached by hardening the seal pattern through a process of UV and heat treatment.
In the process of forming the liquid crystal layer by the conventional vacuum injection method, the injection hole is formed before this process and sealed after this process. On the other hand, in the process of forming a liquid crystal layer by the conventional dispenser method, since the liquid crystal materials are directly dispensed onto the substrate, the forming and sealing processes of the injection hole are not necessary.
However, the process of forming a liquid crystal layer by the conventional dispenser method has some drawbacks. First, in the case of large-sized substrate, the dispensing time becomes long due to an increase in the number of points.
Second, to lessen the dispensing time, if the volume of one droplet of the liquid crystal materials is increased, the uniformity of the volume is hard to maintain. Moreover, if the size of the point is increased, the cell gap does not become uniform due to a reduction of the planarization during the attachment process of the upper and lower substrates and the spots of the display occur due to the damage of the orientation film.
Third, if impurities exist on the substrate or in the liquid crystal materials before the deposition process, these impurities produce partial surface spots between the dispensed region and the other region.